Conventionally, a spectrum analyzer includes a QP detecting (quasi-peak detecting) unit constituted by an analog circuit. The QP detection is a detecting method prescribed by the CISPER standard, and is used to measure the EMC.
The QP detection unit serves as a charge circuit if an output voltage Vo is higher than an input voltage Vi, and serves as a discharge circuit if the input voltage Vi is higher than the output voltage Vo.
A patent document 1 (Japanese Laid-Open Patent Publication (Kokai) No. H5-136833) discloses a circuit which includes analog circuit elements (resistors and capacitors) in a detection circuit, though the circuit does not carry out the QP detection.
However, there is an on-going trend to make a unit which processes an intermediate frequency signal in spectrum analyzers by using digital circuit elements. Thought it is thus desirable to make the QP detection unit by using the digital circuit elements, there poses a problem of how the QP detection unit is constituted by using the digital circuit elements.
It is an object of the present invention to make the QP detection unit by using the digital circuit elements.